Screwdrivers having removable bits for engaging and driving screws into a work-piece are known. These screwdrivers typically have an elongate mandrel to which at one end a bit is removably coupled.
In many screwdrivers, the bit is coupled to the mandrel by threads. For example, in a power screwdriver disclosed in U.S. Pat. No. 4,146,071 to Mueller et al, issued Mar. 27, 1970, the bit has a reduced diameter externally threaded male portion to be received within an internally threaded female socket in the mandrel. The present inventor has appreciated that a threaded coupling has the disadvantage that the mandrel and bits are both expensive and as well render it difficult and time consuming to change the bit.
The power screwdriver of U.S. Pat. No. 4,146,071 utilizes a system in which the head of a screw is located and retained in coaxial alignment with the mandrel and bit by the head of the screw engaging a part-cylindrical guideway member having a diameter approximately equal to the diameter of the head of the screw. In such a configuration, it is necessary that the mandrel and bit be of a sufficiently small diameter that the mandrel and bit may reciprocate axially through the part-cylindrical guideway member. The constraints of the mandrel and bit being of a diameter not greater than the diameter of the screw head, renders replacement of the threaded coupling of the bit to the mandrel with another system difficult.
Other bit to mandrel coupling systems are known in which the mandrel carries a resilient split-ring in a deep groove in a socket in the mandrel. When the bit is inserted into the socket, the split-ring retains the bit in the socket by the split-ring being partially received in a groove about the bit. Such known systems suffer the disadvantage that with repeated use, the split-rings come to fail as by losing their resiliency. Failure of the ring whether resulting in jamming of the bit in the socket or fracture of the split-ring results in expensive replacement of the mandrel since the split-ring is carried by the mandrel.
Insofar as the external diameter of a mandrel must be limited to the diameter of the head of the screw, serious disadvantages arise in the use of known split-ring systems. Firstly, with reducing diameter of the mandrel, the split-ring must be reduced in size. Reducing the size of the split-ring greatly disadvantageously effects the reliability of the split-ring, its consistency in manufacture and increased likelihood of a failure of the coupling system. In systems which the split-ring is carried by the socket, the present inventor has appreciated the disadvantage that the sidewall of the mandrel about the socket must have sufficient radial depth to receive the split-ring totally therein. This requires increased thickness of the mandrel about the socket. Machining the socket to have a groove with a radial depth sufficient to totally receive the split-ring becomes increasingly difficult with sockets of smaller diameter. Using smaller diameter split-rings has the disadvantage that in ensuring a bit is secured against removal, the split-rings must be selected such that forces required to axially withdraw the bits become great due to the variance of the small split-rings when manufactured. Frequently, small diameter split-rings only permit withdrawal of a bit with extremely considerable forces as requiring the use of a vice or plyers or are too easily removed.
A further difficulty with conventional power screwdrivers is that in operation, the head of the screw which is to be driven frequently is misaligned, locating several degrees out of axial alignment with the mandrel and bit. The result is that when the bit is moved to engage the screw head, the screw tends to "cam out", wherein as the screw is driven, it moves further out of axial alignment with the bit and mandrel until the bit can no longer properly engage the screw head. In addition to difficulties in keeping the bit engaged in the screw head, when driving a screw which has moved out of axial alignment, the bit frequently becomes wedged in the slot of the screw head as a result of different axial orientations of the bit and screw. A bit which has become wedged in the screw head may remain jammed in the screw head so as to be withdrawn from the socket on reciprocal upward movement of the mandrel as the screwdriver is positioned to drive the next screw.